In MyMessages_pl.properties is some junk just before first '#' which couses compilation to fail for polish locale .

[ERROR] ﻿# is not a valid resource key as it does not occur in the default version of com/extjs/gxt/ui/client/messages/MyMessages_pl.properties nor in any of []To reproduce:
In module.gwt.xml add:

<set-property name="locale" value="pl_PL" />
<extend-property name="locale" values="pl_PL" /> Workaround :
Copy MyMessages_pl with full path to your project source and remove first character at begining of the file (before '#') . GWT compiler should take corrected version